Polyimide layers are frequently used in the construction of micro-electronic devices and chip packaging structures. A polyimide layer may be formed on a substrate by coating the substrate with a precursor material, generically called polyamic acid, and then curing the precursor material with a heat treatment, which converts the polyamic acid into polyimide and water. The layer of precursor polyamic material may be patterned before or after the heat treatment step.
Polyamic acids generally comprise chains-of repeated organic units. Each organic unit generally has a number of benzene rings (C.sub.6 H.sub.6) linked together in a serial manner by intermediate molecules, such as singlet oxygen (--O--) and carbon-nitrogen pairs (--C--N--). In the carbon-nitrogen pairs, an oxygen atom may be double bonded to the carbon atom, and a hydrogen atom may be single bonded to the nitrogen atom. The chemical formula for one repeated unit of a basic and exemplary polyamic acid is provided below: ##STR1##
This basic structure of polyamic acid may be modified by adding additional atoms or substituting some atoms with different atoms. One common feature of polyamic acids is the location of a carboxyl group (COOH) and a carbon-nitrogen group (--C--N--) in close proximity to one another, often on adjacent positions of a carbon ring as shown above.
Carboxyl groups (COOH), which are commonly found in organic acids, enable the hydrogen atom in the group to be disassociated in water to provide acidic action. Typically, in a polyamic acid, there are two carboxyl groups bonded to two respective carbon atoms of a benzene ring in each repeated unit. The carbon atom in the carboxyl group has a single bond to the respective carbon atom in the benzene ring, a double bond to one of the oxygen atoms, and a single bond to an oxygen atom of an OH group. The polyamic acid, by itself, is a solid which is not readily dissolvable in acidic aqueous solutions. However, in basic solutions, the acid groups react with the alkaline reagents of such solutions, which enables the polymeric acid to more readily dissolve.
When sufficient energy is imparted to the polyamic acid, such as by the application of heat to a temperature above 200.degree. C., the OH group of a carboxyl group (COOH) can react with the hydrogen atom of an adjacent carbon-nitrogen pair (--C--N--) to form a water molecule (H.sub.2 O). In this process, the carbon atom in the decomposed carboxyl group forms a single bond with the nitrogen atom from the carbon-nitrogen pair. The resulting molecular unit is called a polyimide unit and has the following form: ##STR2##
The formation of the single bond between the carbon and nitrogen atoms forms a cyclic ring having a nitrogen atom and a number of carbon atoms, which is known as an imide ring: ##STR3##
Accordingly, the above reaction process is often called "imidization", or "imide-ring cyclization," and is well known in the art. The imidization reaction destroys the acidic action of the carboxyl group, which greatly reduces the solubility of the polyamic acid.
Of course, the carboxyl group (COOH) and the carbon-nitrogen pair (--C--N--) which react together may not be attached to a common benzene ring, but may be attached to separate benzene rings on different polymer chains in close proximity to one another. In this case, a cyclic ring is not formed, but two polyamic acid chains are cross-linked together. Nonetheless, the acidic action of the carboxyl group is destroyed, and the solubility of the polyamic acid is reduced. Because the results are the same as in the previous case, the reaction is often referred to as "imidization" as well. The reactions for carboxyl group decomposition and imidization are commonly initiated by exposing the polyamic acid to temperatures in the range between 300.degree. C. and 450.degree. C. Once the polyamic acid becomes imidized to a substantial degree (usually more than 50% of carboxyl groups being decomposed and re-bonded to carbon-nitrogen pairs), it is referred to as polyimide. As explained in greater detail below, the present invention is related to the etching of polyamic acid, not to the etching of polyimide.
There are many modified forms of polyamic acid currently available which have made various modifications to the repeating organic units in the polymer chains. Nonetheless, all the forms of polyamic acid have carboxyl groups (COOH), or other acidic functional groups, which are capable of decomposing and forming cyclic rings (e.g., imide rings) with respective nearby intermediate molecules of their respective polymer chains, and/or capable of decomposing and forming chemical bonds to the intermediate molecules of other polymer chains (e.g., cross-links to other polymer chains). For the purposes of the present invention, a polyamic acid is defined as a composition having a plurality of polymer chains with different molecular weights (MW) and MW distributions, with each chain having a plurality of acidic function groups (e.g., carboxyl groups), and with at least ten percent of the acidic functional groups on each chain being located near an atom to which it can chemically bond upon decomposition or chemical reaction of the acidic group.
Polyamic acid is often patterned before it is imidized. After a layer of polyamic acid is coated over the substrate, a photoresist layer is often formed over the polyamic acid layer and patterned, which opens up mask apertures over the layer of polyamic acid. The exposed portions of polyamic acid are then wet etched by a suitable basic etchant, which may, for example, comprise tetra-methyl-ammonium hydroxide (TMAH). When using alkaline etchants (e.g., alkaline developers) at moderate concentrations and at room temperature, the inventors have discovered that the exposed portions of polyamic acid can form gelatinous masses (which we call "gels"), particularly for layers thicker than 10 .mu.m or via apertures having high aspect-ratio dimensions, which impair and/or prevent the complete removal of the exposed portions. Under such conditions, via structures formed in the polyamic acid layer sometimes have their bottoms filled with gelatinous masses, particularly if the via apertures have high aspect ratio dimensions or if the polymeric layer is thick.
Conventional thinking in the art holds that if an alkaline etchant or developer cannot effectively remove material from the exposed portions of the material layer, then one must increase the concentration of the etchant and, if necessary, also increase the temperature of the etchant solution, in order to increase the efficacy of the etchant. Unfortunately, the approaches of increasing the concentration and of increasing the temperature add to the cost of processing the substrates. Moreover, each of the approaches are more easily carried out by bath processing rather than spray processing, the former of which is more difficult to control and has greater particle contamination problems. Moreover, highly concentrated etchant solutions are corrosive to many metal materials used in the construction of microelectronic devices and chip packaging structures; thus, the etching of the polymeric layer often corrodes metal layers underlying the polyamic acid layer.
Accordingly, having thus recognized the problem of gels forming in via apertures which are formed in thick layers or with high-aspect ratio dimensions, the inventors see that there is a need in the art to find less-expensive and less-corrosive ways of etching polyamic acid layers and other similar polymeric layers which comprise acidic functional groups, such as carboxyl groups (COOH).